While capacitor monitoring or measuring types of circuits for providing an output upon the change of the capacitance of one leg of a bridge circuit are old such as in the Arlon D. Kompelien U.S. Pat. No. 4,431,962, issued Feb. 14, 1984, such circuits are subject to the stability of the power supply and the relatively constant temperature of the components of the circuit. Enthalpy control devices having mechanical humidity responsive elements and a temperature responsive element have been used in air conditioning systems for years, such as a control device disclosed in the Keith M. Nodolf U.S. Pat. No. 3,949,607, issued Apr. 13, 1976.
The present invention is directed to an electrical circuit for an enthalpy responsive device having a humidity responsive element which changes in capacity in one leg of a bridge circuit and a resistance which changes in response to temperature in another leg. The circuit is fed by a pulsating DC voltage so the circuit output provides a DC signal indicative of enthalpy. To stabilize the output to make it minimally affected by changes in the frequency of the bridge power supply and changes in active amplifier components, a feedback from the output signal back to the bridge circuit is provided by a solid state switch which is closed during one half cycle of the bridge power supply and thus chops the DC output into an appropriate AC signal to be fed into the bridge circuit for stability. A further solid state switch feedback is provided around an intermediate amplifier to provide a reference voltage to a point between the capacitors of separate legs of the bridge circuit during one half cycle of the bridge supply so any signal change during the other half cycle can be properly amplified. This method of providing the capacitors reference voltage cancels effects of input offset voltage changes in the intermediate amplifier while still utilizing its high gain.